It's lossless while still low-ish file size. You probably won't notice if you get a high bitrate mp3 but there will still be compression and quality loss. For example when I bought a song in flac I was able to hear some more subtle background instruments that were too muddy to make out before.
Quick guide:
- MP3 is like jpeg. Supported everywhere, good enough, tiny file size.
- WAV is like bmp. Windows made, gigantic file size, original quality. Supported almost everywhere.
- FLAC doesn't really have a good image comparison. But it's open licence, half the file size of wav, original quality. The only downside is it doesn't have widespread support. It plays fine in almost every dedicated media player like VLC or Evermusic; but if you try to do something like upload it to Discord or play it on the default iPhone music app it won't work.